The present invention relates to a high intensity discharge ballast providing instant restrike of hot high intensity lamps, rapid warm-up and normal ballasting functions.
High intensity discharge (HID) lamps are used in many applications because of their long life and high efficiency for converting electrical energy to light. Unlike incandescent lamps, HID lamps have negative resistance properties and require a ballast to limit their current. Furthermore, these lamps take two to three minutes to reach normal operating temperatures and when extinguished, they take two to five minutes to cool before restarting. In most applications, the time delay during the heating and cooling cycle are acceptable but not desireable. In special applications, such as graphic arts and stadium lighting, for example, the heating and cooling cycles are unacceptable which necessitates better ballast performance to circumvent these deficiencies.
U.S. Pat. No. 3,944,876 issued to James G. Helmuth Mar. 16, 1976 describes Rapid Starting of Gas Discharge Lamps. The invention relates to overcoming problems in starting gas discharge lamps. The two long standing problems addressed by this invention are the objectionable long warm-up times, up to five minutes in certain cases and the practical impossibility of re-starting a warmed up mercury vapor lamp until it cools. For repetitive on or off duty, it has been the practise to leave the lamp on all the time and resort to a mechanical shutter which is moved to control exposure. Such shutters however, are the cause of two major problems: (1) they are subject to mechanical failure; (2) the resultant prolonged exposure to a mercury vapor lamp in the On condition can cause serious damage to the eyes and skin.
To overcome the above problems, the Helmuth invention resorts to the use of a starting device which comprises means to apply to the lamp one or more transient voltage plates which initially change in amplitude in a polarity direction relatively in opposition to the polarity of the main voltage simultaneously supplied to the electrodes and thereafter each pulse is typically caused to change in amplitude in a polarity direction in aid of the polarity of the main voltage then being applied to the electrodes resulting in a rapid start of the lamp. The Helmuth invention further provides circuitry to achieve the above. An additional object of the Helmuth invention is to provide effective ballast circuitry between the main AC source and the lamp shortening the time from start to full light out, without drawing excessive current from the mains and without applying excessive power to the lamp.
U.S. Pat. No. 3,931,544 issued on Jan. 6, 1976 to Ira Jay Pitel relates to Fast Warm Up Electronic Ballast Circuit For A High Pressure Discharge Lamp. This invention describes an electronic ballast circuit for effecting a relatively rapid activation of an HID lamp.